1. Field of the Invention
Isomerization of sym-octahydrophenanthrene (s-OHP) to sym-octahydroanthracene (s-OHA) in the presence of AlCl.sub.3 catalyst is well known. Of particular interest herein are methods for providing increased yields of pure s-OHA and recovery of AlCl.sub.3 catalyst in the isomerization of s-OHP to s-OHA.
2. State of the Art
Anthracene is a starting material in a commercially-viable process for making anthraquinone. In addition to use in preparation of dyestuffs, anthraquinone is being used increasingly as a delignification catalyst in paper pulping processes. Anthracene is found naturally in coal tar at a concentration of about 1 to 3 weight percent of all coal-tar hydrocarbons. About one-half of this amount of anthracene is recoverable by commercial distillation and crystallization methods. Phenanthrene, an isomer of anthracene, is found in coal tar at a concentration of about five weight percent of all coal-tar hydrocarbons. Since phenanthrene is more abundant that anthracene, much attention has been given to conversion of phenanthrene to anthracene.
The only known conversion of phenanthrene to anthracene involves three steps. Firstly, phenanthrene is catalytically hydrogenated to sym-octahydrophenanthrene (s-OHP); secondly, s-OHP undergoes catalyzed isomerization to sym-octahydroanthracene (s-OHA); and thirdly, s-OHA is dehydrogenated to anthracene. In the second-step of isomerization of s-OHP to s-OHA, in the presence of AlCl.sub.3 as the isomerization catalyst, all known isomerization reactions provide relatively low yields of the desired s-OHA isomer or relatively high by-product impurities.
For example, a 1924 German publication (G. Schroeter, Ber. 57B, 1990-2003) discloses a reversible isomerization reaction starting with either pure s-OHP or pure s-OHA isomer. In this reversible reaction, 50 percent yields of both s-OHP and s-OHA are obtained from either starting isomer in the presence of small amounts of AlCl.sub.3 at an isomerization temperature in a range of 70.degree. to 80.degree. C.
In U.K. Pat. No. 694,961 s-OHP is isomerized to s-OHA in the presence of dispersed, finely-divided AlCl.sub.3 catalyst at an isomerization temperature in a range of 5.degree. to 45.degree. C. Dilution of the reaction products with toluene and subsequent hydrolysis with aqueous hydrochloric acid provides a toluene layer containing the s-OHP and s-OHA isomers. Acid hydrolysis precludes reuse of the AlCl.sub.3 catalyst and s-OHA yield ranged from about 70 to 83 weight percent in the presence of about 10 to 13 weight percent unidentified by-products.
A 1978 West German publication [K. Handrick et al., "Production of Anthracene from Phenanthrene," Compend.-Dtsch. Ges. Kohlechem., 78-79(2), 1089-1106] describes a starting mixture containing s-OHP in the presence of about six weight percent s-OHA; after isomerization, the reaction product mixture contains an equilibrium mixture of s-OHP and s-OHA isomers, there being about 50 to 60 weight percent s-OHA present.
Prior art isomerization processes provide relatively low yields of the desired sym-octahydroanthracene isomer or provide relatively large amounts of unwanted by-products. Moreover, where an aqueous-acid hydrolysis step is used to isolate the desired isomer, AlCl.sub.3 catalyst is decomposed and thus rendered ineffective for further catalysis. There is need, therefore, for s-OHP to s-OHA isomerization processes characterized by improved methods for separation and recovery of s-OHA product or AlCl.sub.3 catalyst.